Measurement of the coagulation rate constant for sulfuric acid particles as a function of particle size using tandem differential mobility analysis

A new method for the determination of coagulation rate constants for monodi sperse, neutral particles is described. In this method, a differential mobi lity analyzer (DMA) is used to prepare a monodisperse aerosol and a second DMA is used to separate the coagulation products from the original monodisp erse particles. The experiments are carried out under initial rate conditio ns so that typically 5-9% of the monomer particles undergo coagulation. Exp erimental results at 298 +/- 1 K for H2SO4/H2O particles with diameters of 49-127 nm and a composition of 72-73% H2SO4 by mass gave enhancement factor s, relative to rate constants calculated for hard spheres, that vary from a bout 1.2 for the largest particles to 2.8 for the smallest particles. Fitti ng these results to a theoretical expression accounting for van der Waals f orces gives a Hamaker constant of (6.4 +/- 2.6)x 10(-13) erg. We also give convenient formulas for computing coagulation enhancement factors from the Hamaker constant. (C) 2001 Elsevier Science Ltd. All rights reserved.